Cotton linters have long been recognized as providing cellulose furnishes suitable for the production of premium cellulose derivatives. Raw cotton linters have been considered an excellent source of high molecular weight cellulose for over 80 years. Raw cotton linters, commonly referred to as “linters”, are short fiber residues left on the cottonseed after the longer staple (“lint”) fibers are removed by ginning. Linters are shorter, thicker, and more colored fibers than staple fibers. Linters are removed from cottonseeds using a number of technologies including linter saws and abrasive grinding methods, both of which yield suitable materials. Depending on the number of passes used to remove the linters from the cottonseed, they are called “first-cut”, “second-cut” and “third-cut” raw cotton linters. If the linters are removed in one pass or first- and second-cut linters are manually blended in a weight ratio of approximately 1:4, the resulting material is called “mill runs”. Mill runs and first-cut raw cotton linters are used in medical and cosmetic applications as well as to make upholstery, mattresses, etc. while second cut cotton linters are typically used to manufacture purified cotton linters or chemical cotton. In general, first-cut cotton linters contain less non-cellulosic impurities than do second-cut cotton linters. The amount of hemicellulose, lignin or colored impurities and foreign matter in the various types of raw cotton linters increases in the following order: First-cut<second-cut<third-cut. Typically, the cellulose content of raw cotton linters is about 69-78 wt % as measured by the American Oil Chemists' Society (AOCS) “bB 3-47: Cellulose Yield Pressure-Cook Method”.
In the past, the use of cotton linters in chemical processes was only after extensive mechanical and chemical cleaning to yield a high purity furnish. Purified cellulose obtained from raw cotton linters is called chemical cotton or purified cotton linters. Given the commercial significance of cotton, it is not surprising that many mechanical separation processes have been developed over the past century to separate lint and linters from other contaminants. In contrast to the present invention these previous methods have utilized uncut fibers make the separation process very difficult due to a number of factors which will be discuss in more detail.
Until recently linters obtained from cottonseed oil producers underwent extensive dry cleaning as well as chemical purification prior to their use in chemical reactions. A previous invention (U.S. Ser. No. 10/822,926, Gillette et al., Filed Apr. 13, 2004 entitled “Improved Raw Cut Cotton Linters, Method of Making, and Uses Thereof”) demonstrated that cellulose derivatives suitable for a variety of applications could be produced from unpurified “raw” linters. For a number of applications, such as those that are color sensitive or command the highest molecular weight products, it is desirable to perform some purification of the raw cotton linters. Additional benefits from partial purification or cleaning of raw linters may be realized during cellulose ether manufacture. These include, but are not limited to, more efficient use of derivatizing reagents and reduced contamination of both slurry process diluent liquids as well as purification solvents.
Heretofore suppliers of highly purified linters have sought to preserve linters fiber length as much as possible since this attribute is desirable for many of their customer's applications. Several examples of equipment designed to clean raw cotton linters are presented in (U.S. Pat. No. 661,166, R. R. Boyd, filed Nov. 6, 1990, entitled “Process of Treating Cotton-Waste”; U.S. Pat. No. 2,004,731, C. H. R. and A. J. V Ware, filed Jun. 11, 1935, entitled “Means and Method for Defibrating”; U.S. Pat. No. 2,210,016, A. J. V. Ware, filed Aug. 6, 1940, entitled “Cleaner for Cotton Linters”; U.S. Pat. No. 2,632,924, L. L. Bertram et al., filed Mar. 31, 1953, entitled “Apparatus for Cleaning Fibrous Materials; and U.S. Pat. No. 3,815,178, M. A. Goldman, filed Jun. 11, 1974, entitled “Cotton Linter Refining Process and Apparatus”). Examples of commercially available equipment are the Carver Hi-Cel.™ linter cleaner and the Continental Eagle Industrial Metal Products LC-410D lint cleaner. In addition, the suppliers of highly purified linters typically provide their product in heavy gauge paper sheet form, the production of which is greatly facilitated by maintaining linters fiber length. As will be discussed, long fibers are not only unnecessary, but also undesired for the production of cellulose derivatives.